Your diet is probably dangerously acidic but there’s a simple solution

I am standing in the bathroom with a strip of litmus paper in my hand. I am going to pee on it and hope that it doesn’t turn red, which would indicate acid. This isn’t for a bet – it is a (ahem) litmus test of whether my diet is slowly killing me. Acidic urine is a crude sign that something called my dietary acid load is too high. If it is, I am opening myself up to a range of ills. Luckily, there is a simple cure: a change of diet. So, if I do see red, I am going to eat some spinach and try again. 

This might sound like medical woo-woo, and there are worrying echoes of a discredited fad called the alkaline diet. But nutrition scientists increasingly think that by ignoring dietary acid load we are missing a trick when it comes to healthy eating. “The higher the dietary acid load, the higher the risk of developing chronic diseases,” says Hana Kahleova at the Physicians Committee for Responsible Medicine, a not-for-profit research centre in Washington DC. These include kidney disease, liver disease, cancer, obesity, hypertension and even anxiety and depression. 

Shockingly, almost all of us are getting this wrong – especially if we eat a regular Western diet. But the good news is that, unlike the damage caused by consuming too much salt or more calories than we need, this can be quite easily reversed, provided you know which foods and drinks make your body too acidic. What’s more, the new science of dietary acid load is throwing fresh light on why certain diets promote chronic diseases. 

The idea that the food we eat affects the acid-alkaline balance of our bodies was established in the 1960s when doctors discovered that, although human urine is usually acidic, the pee of vegetarians tends to be slightly alkaline. This was later shown to be correlated with the amount of “acid ash” in people’s diets. A now-obsolete measure, the acid-ash test entails incinerating foodstuffs and analysing the resulting ash, a process that supposedly mimics metabolism and indicates whether the final breakdown products of digested foods are acidic or alkaline.

In 1968, two doctors at Harvard University proposed that too much dietary ash was the cause of two major diseases of old age, osteoporosis, or bone loss, and sarcopenia, or muscle wasting. Their hypothesis was that to buffer the effect of excess acid, bones and muscles are broken down to release alkaline compounds such as carbonates, phosphates and ammonia. The result, they proposed, is a reduction in bone density and muscle mass. The acid-ash hypothesis has long fallen out of favour, largely because there is no evidence that an acidic diet is a risk factor for osteoporosis. However, the basic idea lives on in the form of dietary acid load – a phoenix from the flames.  

Measuring acidity is Chemistry 101. You probably remember using litmus paper in a school science lab to assess the pH of various substances on a scale from 0 to 14, with 7 being neutral, anything below being acidic, and anything above being alkaline. As with my pee test, the paper will turn red to indicate acid and blue for alkali. A more sophisticated version of this test reveals that the pH of human blood and the intracellular fluids derived from it is kept on a tight leash – ideally within the range of 7.35 to 7.45, so slightly alkaline.  

“Our body needs to keep the pH very constant,” says Kahleova. Stray outside this and things go quickly downhill, especially if it drops below the lower limit. This is a state called acute metabolic acidosis, which manifests as a rapid heartbeat, confusion, fatigue, nausea, dizziness, headaches and, in extreme cases, death. Nasty, but rare. Normally, our bodies have little difficulty staying on their pH leash. Most cases of acute acidosis are caused by underlying conditions such as kidney and liver disease, cancer and diabetes, although it is not unknown in people eating an extremely high-protein diet, exercising to exhaustion, suffering from acute diarrhoea or overdosing on laxatives.  

What makes me acidic?

The two main sources of acid in the bloodstream are respiration, which generates carbon dioxide (which becomes carbonic acid when dissolved in water) and the digestion and metabolism of food and drink, which produces many other acidic compounds. The lungs deal with the former and the kidneys the latter. Carbon dioxide doesn’t cause acidosis, because the lungs easily excrete it. But, depending on your diet, acids derived from things you consume can: the overall balance as a result of food and drink is referred to as the dietary acid load, or DAL.  

To maintain its preferred mildly alkaline state, our body must excrete the same amount of acid as it gains. When acid predominates, the kidneys filter out the excess and dump it into the urine. If more must be done to get back on track, they also retrieve alkaline bicarbonate ions from the filtrate and return these to the bloodstream. For most people, most of the time, this system is perfectly capable of keeping their body in the Goldilocks zone. Unfortunately, staying out of acute metabolic acidosis isn’t enough, though. Even hovering on or around the lower pH limit of 7.35 can cause problems, pushing us into a state called low-grade metabolic acidosis, which, despite not being as dangerous as acute acidosis, is still a health risk.

What determines whether a food is acidic isn’t its pH when it is on your plate, but the pH of its metabolites. The final breakdown products of what we eat and drink range enormously in pH, from quite acidic through to quite alkaline. They can be surprising. Many relatively acidic foods, such as citrus fruits, are actually alkaline in the context of DAL because the abundant citric acid they contain is metabolised to bicarbonate, which is alkaline. They, and other fruits and vegetables, are also rich in proteins that produce alkaline metabolites. This is in sharp contrast to animal proteins. They are rich in the sulphur-containing amino acids cysteine, homocysteine and methionine, plus three other amino acids, namely lysine, arginine and histidine, all of which lead to acidic metabolites. Many grains and nuts contain these acid-generating amino acids too. Indeed, proteins are the main determinant of DAL. Other notable sources of acid in our diets are the chloride ions in table salt (sodium chloride) and the food additive phosphoric acid, which is put into fizzy drinks and a wide variety of processed fare such as meats, dairy products and cereal bars.  

More perils of a Western diet

At this point, alarm bells may be ringing. Western-style diets are notoriously rich in animal products, salt, refined grains and ultra-processed foods, and low in fruit and vegetables – the perfect recipe for low-grade metabolic acidosis. Indeed, researchers believe that among people consuming the typical Western diet, it is very common, if not ubiquitous. “We have a chronic exposure to a high dietary acid load, so that’s something that we all have,” says Ilias Attaye at Erasmus University Medical Center in Rotterdam, the Netherlands. 

But addressing this isn’t as straightforward as simply switching away from a Western diet – there are many factors to take into account. One is that some fresh fruits and vegetables contain compounds that are metabolised to oxalic acid, which pushes them towards the acidic column. This makes things like beets, blackberries, cherries, grapes and raspberries much less alkaline than you might hope, according to Gabriela Leal-Escobar at the Ignacio Chávez National Institute of Cardiology in Mexico City. Many plant-based processed foods, meanwhile, include acid-forming additives such as phosphoric acid. “You have to be very careful about additives, make sure vegetables don’t have anything added to them, because that can really promote the acid load,” says Attaye. 

Another problem is that assessing DAL is notoriously difficult. The pH of your urine is too crude a measure to be of clinical use. There is no gold standard measure for DAL, but there is a method to calculate it more accurately. Potential renal acid load, or PRAL, was developed in the early 1990s to replace the acid-ash test. It is calculated with an equation that estimates the amount of acid or alkali that will be produced when 100 grams of a given foodstuff or drink are metabolised, measured in a unit called milliequivalents per litre (mEq/L). The outcome is a number ranging from about minus 15 to around plus 35. In contrast with the pH scale, however, the more negative the PRAL score, the more alkaline the food; the more positive, the more acidic. In this case, 0 is neutral. 

PRAL is calculated from the content of just five nutrients: total protein, phosphorus, calcium, magnesium and potassium. Protein and phosphorus add to the score, while calcium, magnesium and potassium subtract from it. That may seem plain wrong, as proteins are the major determinant of DAL and can be acid or alkali-producing. But this is compensated for by the fact that animal products contain more phosphorus than plant products do, which, in turn, are richer in calcium, magnesium and potassium, says Kahleova. “So, if you’re consuming more animal protein, automatically your dietary acid load will be higher.”

In general, the mEq/L of an animal-derived food is acidic and plant products are alkaline. “Meat’s dietary acid load is roughly between 8 and 10. Cheese is even more acidic, at around 30 – Parmesan being the highest at 34,” says Kahleova. “Rye bread is about 4 – still slightly acidic. Legumes tend to be around 0 or slightly negative, and most vegetables and fruits are in the negative numbers, like minus 4 or 5. The champion is the leafy greens, at minus 14. So leafy greens are the most effective way to make your diet more alkaline.” Alcoholic drinks, meanwhile, are effectively neutral. Wine has a PRAL score of +0.03 per 100 ml, spirits +0.11 and beer -0.2.  

PRAL can be adjusted to take account of people’s height and weight, but it isn’t a perfect measure. A major weakness is that it doesn’t include salt, so it probably underestimates the true DAL. However, the assumption is that the chloride ions that salt produces are mostly consumed in processed foods, many of which have PRAL scores that take account of salt. Also, since people on Western diets tend to eat roughly the same amount of salt, it has a similar impact on everyone and so can be ignored. Nevertheless, for everyday purposes, PRAL can give a pretty good sense of whether your diet has an acidic or alkaline outcome, and to what extent – and hence offer guidance about healthier eating.

If you are interested in the acidity question, you don’t have to calculate PRAL from scratch. There are tables available containing the scores of hundreds of common foodstuffs, so all you need to do is make a note of what you ate and how much, then tally your PRAL score per day, which is expressed in units of mEq/d. If the number that pops out is below 60, you are almost certainly fine. An overall negative score – unlikely, given the Western diet is overwhelmingly acidic – is also nothing to worry about. Although there is an alkaline version of acidosis, called alkalosis, there are no known cases of it being caused by an excessively alkaline diet, according to Kahleova. However, if your score is over 60 mEq/d, that is a problem. 

I did this for a few days – not easy, as the tables only feature raw ingredients – and found that my PRAL figure was consistently around 70 mEq/d. That is on the low side of what is typical for somebody eating a Western-style diet. But then, I am vegetarian – albeit one with a passion for cheese. My score still isn’t good, though. Attaye notes that healthy kidneys can eliminate between about 40 and 60mEq of acid per day without any trouble. Admittedly, they can deal with a lot more, but that takes a toll. “They will always find a way to keep the pH where it’s supposed to be. But it places a huge demand on them,” says Kahleova. Overloaded kidneys work hard to raise the pH above the lower threshold – but only just. The result is low-grade metabolic acidosis, and its attendant health problems. 

The first organ system to feel the burn is the kidneys themselves. Persistent overwork gradually grinds them down, leading to mild chronic kidney disease. This then starts a vicious cycle, says Attaye. Diseased kidneys are less efficient at eliminating excess acid, so have to work ever harder to maintain a healthy pH. Eventually, they can’t keep up and low-grade metabolic acidosis can develop into full-blown acidosis. People with kidney disease are already often advised to eat a low-PRAL diet. But it is probably better for your kidneys to take action before the problems start. “The lighter we can make their work, the better for us,” says Kahleova. 

How acid corrodes you

The link between a high DAL and kidney disease is pretty much nailed down. Now, there is a growing suspicion among nutrition scientists that the acid inside our bodies eats deeper. Low-grade metabolic acidosis has been tentatively linked to multiple chronic conditions, including diabetes, obesity, liver disease, cardiovascular disease, hypertension, cancer, anxiety and depression. As yet, these are just associations from small-scale studies. Bigger ones are needed before DAL can be built into dietary guidelines, says Attaye. But they are coming. For example, he has just received funding for a clinical trial to look at how low and high-acid diets affect the metabolic health of people with diabetes.

The link with obesity is quite robust too. Kahleova recently completed a trial in which overweight adults followed either a Mediterranean-style diet or a low-fat vegan diet for 16 weeks, then swapped over. Participants could eat as much as they wanted as long as they didn’t eat the wrong things. The Mediterranean diet had a negligible impact on weight loss, but when on the vegan diet, participants lost an average of 6 kilograms of body fat. That was mainly because they consumed fewer calories – but this only accounted for about three-quarters of the weight loss. Kahleova believes DAL accounts for the rest. She measured the acidity of the diets using PRAL, with an adjustment for weight and height, and found that, although both were alkaline, the vegan one was much more so: its PRAL score was around -19.3 mEq/d, compared with -1.6 mEq/d for the Mediterranean diet. “The alkaline environment basically enables metabolic processes to run more efficiently,” she says. “The vegan diet increases metabolism, so after each meal the body just burns more calories.” 

Other conditions that have been linked to high DAL need further testing. Nevertheless, Attaye sees a plausible mechanism by which an excessively acidic diet might lead to chronic diseases. “The fundamental studies are lacking, so we don’t really understand what’s going on,” he says. “But my thought is that it contributes to low-grade inflammation.” Overloading the kidneys causes the release of the stress hormone cortisol, which leads to inflammation. And chronic inflammation is a known risk factor for multiple conditions.  

Given the known and suspected consequences of a high-acid diet, perhaps it is no surprise that it also increases the risk of premature death. Earlier this year, a team led by Mohammad Reza Pashaei at Urmia University of Medical Sciences in Iran published a review of the literature on DAL. The researchers found that each increase of 10 mEq/d raises the risk of dying from any cause by 3 per cent. 

All of this is pointing to a way of eating healthily that both Attaye and Kahleova believe will become as influential and well-known as the Mediterranean diet: the low-acid diet. To be clear, it isn’t the same as the fad alkaline diet, which rests on the unscientific assumption that too much acid causes cancer and that an alkaline diet can prevent and cure it. This has been debunked repeatedly. In 2018, the British Dietetic Association went so far as to declare it as “nonsense”. 

A scientifically sound low-acid diet wouldn’t be radically different from existing healthy eating guidelines. A focus on DAL strongly backs the message that cutting down on animal products, salt, refined grains and ultra-processed foods is good for you – as is eating more fruit and vegetables. “It reinforces the more general concept of eating less animal protein, less processed foods and more leafy greens,” says Attaye. “But there are some nuances. It’s not completely the same because also some vegetables and grains have a high acid load.” 

We have long understood that what we eat can contribute to chronic diseases. Our emerging understanding of DAL is bringing new insights into how these two things are linked. “The field of nutrition is working towards a more holistic view of food. DAL, I think, is one of the ways to look at it. It’s not the only way, but it’s one of the ways,” says Attaye.  

Better yet, there is an immediate win we can all take from this. The damage we do to ourselves by consuming too much acid-forming food and drink can quite easily be reversed by choosing alkali-forming alternatives. I can attest to this. My urine test did come up red, but after eating a bag of spinach, I tried again and this time the litmus paper was blue.  

“This is something we can influence through dietary choices,” says Kahleova. “It’s a simple, simple intervention that everybody can do.”